Grease composition, rolling bearing, and motor

ABSTRACT

A grease composition including: a base oil, a thickener, and 0.2 wt % or more of a dispersing agent. The dispersing agent includes at least one of a metal salt of a carboxylic acid, a metal salt of a polycarboxylic acid, and a copolymer of a carboxylic acid metal salt and a hydrocarbon compound, and the dispersing agent has an average particle diameter of 0.5 μm or more and 5 μm or less. The metal salt of the carboxylic acid, the metal salt of the polycarboxylic acid, and the carboxylic acid metal salt forming the copolymer of the carboxylic acid metal salt and the hydrocarbon compound may be an alkali metal salt including at least one of a sodium salt, a lithium salt and a potassium salt, or an alkali earth metal salt including at least one of a magnesium salt and a calcium salt.

CROSS-REFERENCE TO RELATED APPLICATION(S)

The present application claims priority to and incorporates by reference the entire contents of Japanese Patent Application No. 2017-012863 filed in Japan on Jan. 27, 2017.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a grease composition, a rolling bearing, and a motor.

2. Description of the Related Art

A grease composition is used for lubrication in a rolling bearing applied for a fan motor and the like of consumer electronics products and medical devices. The grease composition contains a base oil, a thickener, and additives. Examples of the grease composition include a urea grease containing a urea compound as the thickener. The urea grease has excellent heat resistance and thus a rolling bearing using the urea grease shows excellent acoustic property even when the rolling bearing is used at a high temperature.

For example, Japanese Patent Application Laid-open No. 2004-224823 discloses a urea grease containing a urea compound as the thickener and at least one of a carboxylic acid, a carboxylic acid salt, and a carboxylic acid ester as the additive (a rust inhibitor) in the grease composition containing the base oil, the thickener and the additive.

Japanese Patent Application Laid-open No. 2004-339448 discloses a urea grease containing the base oil containing 30% by mass (wt %) or more of an aromatic ester oil and containing 5% by mass to 35% by mass (wt %) of a diurea compound in the grease composition containing the base oil, the thickener and the additive.

For a fan motor used in a consumer electronics product and a medical device, the temperature in the use environment has become higher due to an increase in the heat quantity associated with high speed rotation. Along with this, the temperature in the use environment of the rolling bearing has also become higher. Consequently, the urea grease is required to reduce thermal deterioration even when the grease is used in a temperature higher than the temperature in the usual environment. In addition, quietness is particularly required for the medical device from the initial stage to the end of service life. The conventional urea grease, however, has insufficient heat resistance and thus the grease gradually coagulates under such high temperature environments to deteriorate the acoustic property when the grease is used for a rolling bearing. Consequently, further improvement is also required for the acoustic property.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above problems and an object of the present invention is to provide a grease composition that causes less thermal deterioration when the grease composition is used under a high temperature environment for a long period, and a rolling bearing and a motor that have excellent acoustic property from the initial stage when the rolling bearing and the motor are used under the high temperature environment.

A grease composition according to an embodiment includes a base oil, a thickener and 0.2 wt % or more of a dispersing agent, wherein the dispersing agent comprises at least one of a carboxylic acid metal salt, a metal salt of a polycarboxylic acid, and a copolymer of a carboxylic acid metal salt and a hydrocarbon compound, and the dispersing agent has an average particle diameter of 0.5 μm or more and 5 μm or less.

The above and other objects, features, advantages and technical and industrial significance of this invention will be better understood by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view illustrating the fan motor including a rolling bearing according to an embodiment of the present invention;

FIG. 2 is a cross-sectional view illustrating the rolling bearing according to the embodiment of the present invention;

FIG. 3 is a graph illustrating the experimental result of an example of the present invention;

FIG. 4 is a graph illustrating the experimental result of an example of the present invention; and

FIG. 5 is a graph illustrating the experimental result of an example of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the grease composition, the rolling bearing, and the motor according to the present invention will be described in detail with reference to the attached drawings. The present invention, however, is not limited by the following embodiments. In each drawing, the same sign will be adequately assigned to the same or corresponding element and the overlapped description will be adequately omitted.

FIG. 1 is a cross-sectional view illustrating a fan motor 10 including a rolling bearing 20 according to an embodiment of the present invention. FIG. 2 is a cross-sectional view illustrating the rolling bearing 20 according to the embodiment of the present invention.

The fan motor 10 according to the embodiment of the present invention includes a rotor shaft 11, rolling bearings 20, a stator 12, an impeller 13, and a casing 16. The rotor shaft 11 is held so as to be capable of being rotated by the rolling bearings 20. The impeller 13 has a rotor housing 14 and blades 15 provided on the outer periphery of the rotor housing 14.

The rolling bearing 20 includes an inner ring 21, an outer ring 22, spherical rolling elements 23, retainers 24, and sealing members 25. A grease composition G is encapsulated in the inside part sealed by the sealing members 25. The rolling bearing 20 according to the embodiment of the present invention may be applied not only to a fan motor but also to other types of motors.

Subsequently, the grease composition G according to the embodiment of the present invention will be described. The grease composition G according to the embodiment of the present invention contains a base oil and a thickener, and further contains at least one of a metal salt of a carboxylic acid, a metal salt of a polycarboxylic acid, and a copolymer of a carboxylic acid metal salt and a hydrocarbon compound as a dispersing agent. As a result of the intensive studies on the grease composition, the inventors of the present invention have confirmed that, when the rolling bearing to which the grease composition is applied is used under a high temperature environment (for example, 140° C. or higher) over a long period of time, the acoustic property of the rolling bearing deteriorates by coarsening the agglomerates in the grease composition. The inventors of the present invention have also found that the coarsening of agglomerates can be reduced even when the rolling bearing is used under the high temperature environment over a long period of time and thus the deterioration of the acoustic properties can be reduced by including at least any of the metal salt of a carboxylic acid, the metal salt of a polycarboxylic acid, and the copolymer of a carboxylic acid metal salt and a hydrocarbon compound as the dispersing agent in the grease composition. In addition, the inventors of the present invention have found that when the average particle diameter of the dispersing agent is set to 0.5 μm or more and 5 μm or less, the acoustic properties is improved from the initial stage even when the amount of the dispersing agent added is small. The grease composition G according to the embodiment of the present invention is made based on this finding.

The type of the base oil is not particularly limited. Synthetic hydrocarbon oils, alkyl ether oils, alkyl diphenyl ether oils, ester oils, mineral oils, fluorine oils, silicone oils, and other oils, which are generally used as the grease base oils, can be used singly or in combination. The content of the base oil may be, for example, 70 wt % or more and 90 wt % or less.

Examples of the synthetic hydrocarbon oils may include poly-α-olefins such as normal paraffin, isoparaffin, polybutene, polyisobutylene, a 1-decene oligomer, and an oligomer of 1-decene and ethylene. Examples of the ester oils may include diester oils such as dibutyl sebacate, di-2-ethylhexyl sebacate, dioctyl sebacate, dioctyl adipate, diisodecyl adipate, ditridecyl adipate, and ditridecyl glutarate, aromatic ester oils such as trioctyl trimellitate, tri-2-ethylhexyl trimellitate, tridecyl trimellitate, tetraoctyl pyromellitate, and tetra-2-ethylhexyl pyromellitate, polyol ester oils such as trimethylolpropane caprylate, trimethylolpropane pelargonate, pentaerythritol 2-ethyl-hexanoate, and pentaerythritol pelargonate, and carbonic acid ester oils. Examples of the alkyl diphenyl ether oils may include mono alkyl diphenyl ethers, dialkyl diphenyl ethers, and poly alkyl diphenyl ethers. Among the above-described base oils, the aromatic ester oils are preferable and can be used singly or in combination. In particular, a base oil prepared by mixing tri-2-ethylhexyl trimellitate and tetra-2-ethylhexyl pyromellitate is preferably used.

As the thickener, non-urea compounds and urea compounds can be used. From the viewpoint of the heat resistance and the acoustic property, the urea compounds are preferably used. The content of the thickener may be, for example, in a range of 10 wt % or more and 30 wt % or less.

Examples of the non-urea compound may include metal soaps and polytetrafluoroethylene resins. The metal soap is synthesized from an aliphatic monocarboxylic acid such as stearic acid or an aliphatic monocarboxylic acid containing at least one hydroxyl group such as 12-hydroxystearic acid and an alkaline earth metal hydroxide. A complex metal soap synthesized from an aliphatic monocarboxylic acid and a dibasic acid such as an aliphatic dicarboxylic acid can also be used.

As the urea compound, urea compounds such as diurea compounds, triurea compounds, and polyurea compounds can be used. In particular, from the viewpoint of the heat resistance and the acoustic property (quietness), the diurea compounds are preferably used. The diurea compounds can be represented by following Formula (1).

R₁—NHCONH—R₂—NHCONH—R₃   (1)

where R₁, R₂, and R₃ are hydrocarbon groups selected from aliphatic hydrocarbon groups, alicyclic hydrocarbon groups, and aromatic hydrocarbon groups. Specifically, R₁, R₂, and R₃ may be the aliphatic hydrocarbon groups, the alicyclic hydrocarbon groups, or the aromatic hydrocarbon groups and the carbon number is not particularly limited. R₂ is preferably an aromatic hydrocarbon group and more preferably a mono-substituted or di-substituted phenyl group. As raw materials used at the time of the synthesis of these urea compounds, an amine compound and an isocyanate compound are used. As the amine compound, aliphatic amines represented by hexylamine, octylamine, dodecylamine, hexadecylamine, octadecylamine, stearylamine, and oleylamine, cycloaliphatic amines represented by cyclohexylamine, and, in addition, aromatic amines represented by aniline, p-toluidine, and ethoxyphenylamine are used. As the isocyanate compound, phenylene diisocyanate, tolylene diisocyanate, diphenyl diisocyanate, diphenylmethane diisocyanate, octadecane diisocyanate, decane diisocyanate, and hexamethylene diisocyanate are used. Aliphatic-aromatic diurea compounds synthesized from an aliphatic amine and an aromatic amine used as the amine raw materials and an aromatic isocyanate are preferably used.

Although the carboxylic acid used in the metal salt of the carboxylic acid contained as the dispersing agent is not limited, an aliphatic carboxylic acid is preferable. The aliphatic carboxylic acid may be a saturated aliphatic carboxylic acid or an unsaturated aliphatic carboxylic acid. Also, the polycarboxylic acid used in the metal salt of the polycarboxylic acid is not limited and the aliphatic carboxylic acid is preferable for a carboxylic acid constituting the polycarboxylic acid. The aliphatic carboxylic acid may be a saturated aliphatic carboxylic acid or an unsaturated aliphatic carboxylic acid. As the carboxylic acid constituting the polycarboxylic acid, the unsaturated aliphatic carboxylic acids are more suitably used. Examples of the unsaturated carboxylic acids include acrylic acid, crotonic acid, isocrotonic acid, 3-butenoic acid, methacrylic acid, angelic acid, tiglic acid, 4-pentenoic acid, 2-ethyl-2-butenoic acid, 10-undecenoic acid, and oleic acid. Examples of the saturated dicarboxylic acids include oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, and sebacic acid. Examples of the saturated carboxylic acids include formic acid, acetic acid, propionic acid, butyric acid, valeric acid, caproic acid, enanthic acid, caprylic acid, pelargonic acid, capric acid, lauric acid, myristic acid, palmitic acid, margaric acid, and stearic acid. Examples of the unsaturated dicarboxylic acids include fumaric acid, maleic acid, and itaconic acid. The polycarboxylic acid constituted by these carboxylic acids may be a polymer of a monocarboxylic acid or a polymer of a dicarboxylic acid. In particular, polymers of an unsaturated carboxylic acid having one or two carboxy groups are preferable. The metal salts of these carboxylic acids may form copolymers with hydrocarbon compounds. In other words, the grease composition may contain a copolymer of a carboxylic acid metal salt and a hydrocarbon compound as the dispersing agent. The grease composition may also be constituted by containing at least any of a carboxylic acid metal salt, a metal salt of the polycarboxylic acid, and a copolymer of the carboxylic acid metal salt and the hydrocarbon compound. Specific Examples of the carboxylic acid metal salt forming the carboxylic acid metal salt, the metal salt of the polycarboxylic acid or the copolymer of the carboxylic acid metal salt and the hydrocarbon compound include at least one salt selected from alkali metal salts and alkali earth metal salts. As the alkali metal salts, sodium salts, lithium salts, potassium salts, and the like are preferable. As the alkali earth metal salts, magnesium salts, calcium salts, and the like are preferable. Examples of the hydrocarbon compound for polymerizing (polymerization reaction) with the carboxylic acid metal salt in the copolymer of the carboxylic acid metal salt and the hydrocarbon compound include isobutylene, propylene, isoprene, and butadiene.

As the weight average molecular weight of the polycarboxylic acid, the weight average molecular weight (Mw) in terms of polyethylene glycol measured by gel permeation chromatography (GPC) is preferably 5,000 or more and 200,000 or less, more preferably 7,000 or more and 80,000 or less, and further preferably 9,000 or more and 16,000 or less.

In the grease composition G according to the embodiment of the present invention, the carboxylic acid metal salt or the metal salt of the polycarboxylic acid as the dispersing agent has an average particle diameter of 0.5 μm or more and 5 μm or less and is preferably contained in an amount of 0.2 wt % or more. The carboxylic acid metal salt or the metal salt of the polycarboxylic acid having an average particle diameter of 0.5 μm or more and 5 μm or less can sufficiently improve heat resistance and reduce thermal deterioration even when the content is 0.2 wt %, which is relatively small amount, and thus the excellent acoustic property is obtained. When the dispersing agent is the copolymer of the carboxylic acid metal salt and the hydrocarbon compound, the copolymer is preferably contained in an amount of 0.2 wt % or more. In the grease composition G according to the embodiment of the present invention, the content of the dispersing agent is preferably 5 wt % or less. When the content of the dispersing agent having an average particle diameter of 0.5 μm or more and 5 μm or less is 5 wt % or less, the thickening effect of the grease is reduced and thus an increase in the rotational torque of the rolling bearing can be prevented. In this embodiment, the average particle diameter of the dispersing agent is a value determined by a measurement method of laser diffraction scattering in accordance with JIS Z 8825.

The grease composition G can contain antioxidants, anti-friction agents, metal deactivators, rust inhibitors, oiliness agents, viscosity index improvers, and the like as other additives, if needed.

In the grease composition, the rolling bearing, and the motor according to this embodiment constituted as described above, the grease composition contains the above-described dispersing agent and thus the grease composition has high heat resistance, exhibits less thermal deterioration even the grease composition is used under high temperature environment, and has excellent acoustic property from the initial stage when the rolling bearing is used. The grease composition can also reduce coarsening of the agglomerates even when the grease composition is used under a high temperature environment over a long period of time because the grease composition contains the dispersing agent and thus the excellent acoustic property can be maintained even after the grease composition is used under high temperature for a long period in the rolling bearing.

Those constituted by appropriately combining each component described above are also included in the present invention. Further modified examples can be readily derived by those skilled in the art. The scope of the present invention, therefore, is not limited to the above embodiments and can be modified in various ways.

For example, in the above embodiment, the grease composition can contain a sodium salt of a carboxylic acid or a polycarboxylic acid as a dispersing agent. The constitution of the grease composition may contain a metal salt of a polycarboxylic acid other than the sodium salt of a carboxylic acid or a polycarboxylic acid as a dispersing agent. Examples of the metal salt of a carboxylic acid or a polycarboxylic acid may include a lithium salt, a potassium salt, a magnesium salt, and a calcium salt of a polycarboxylic acid in addition to the sodium salt. In this case, the grease composition contains at least one of the metal salts of carboxylic acids and polycarboxylic acids. When the grease composition contains the metal salt of a carboxylic acid or a polycarboxylic acid, a similar effect to the grease composition G described above is exhibited. The copolymer made of a carboxylic acid metal salt and a hydrocarbon compound may have a constitution using a sodium salt or a metal salt other than a sodium salt. When the grease composition contains two or more of the carboxylic acid metal salt, the metal salt of the polycarboxylic acid, and the copolymer made of a carboxylic acid metal salt and a hydrocarbon compound as the dispersing agents, the dispersing agents preferably have an average particle diameter of 0.5 μm or more and 5 μm or less and are contained in an amount of 0.2 wt % or more in total.

EXAMPLES AND COMPARATIVE EXAMPLES

Subsequently, Examples and Comparative Examples of the present invention will be described. In Examples and Comparative Examples, various grease composition were prepared and the properties of the grease composition were studied.

The base oil, the thickener, and the dispersing agent were mixed using the components listed in Table 1 to prepare the grease compositions of Examples 1 to 12 and Comparative Examples 1 to 7. As the base oil, the ester oil (TOTM (trioctyl trimellitate)+TOPM (tetraoctyl pyromellitate)), PAO (poly-α-olefin), or a mineral oil was used. As the thickener, the mixture of an aromatic urea compound and an aliphatic urea compound as the diurea compound A (Diurea A in Table 1) or the mixture of an aliphatic urea compound and an alicyclic urea compound as the diurea compound B (Diurea B in Table 1) was used. As the dispersing agent, the sodium salt of polyacrylic acid as the metal salt of the polycarboxylic acid (A in Table 1), the copolymer of a maleic acid sodium slat and isobutylene as the copolymer of the carboxylic acid metal salt and the hydrocarbon compound (B in Table 1), or a sebacic acid sodium salt as the carboxylic acid metal salt (C in Table 1) was used. In Table 1, figures listed in percentage represent the content of the dispersing agent in % by mass. More specifically, the grease compositions of Examples 1 to 12 and Comparative Example 1 to 7 contained the dispersing agent A, B, or C in a range of 0.2 wt % to 5.0 wt %. Here, the average particle diameters of the dispersing agents of Example 1 to 12 were set to 0.5 μm or more and 5.0 μm or less, whereas the average particle diameters of the dispersing agents of Comparative Example 1 to 7 were set to 6.0 μm or more and 10.0 μm or less. The average particle diameters of the dispersing agents were determined with Laser Particle Size Analyzer (model number: LA-920, manufactured by HORIBA, Ltd.) using wet dispersion in a measurement method of laser diffraction scattering in accordance with JIS Z 8825. Acetone was used as a solvent and the sample was dispersed before the measurement by ultrasonic sound for 5 minute. For each of the prepared grease compositions, the initial Anderon value described below and the viscosity after a stationary heating test were measured. The initial Anderon value and the Anderon value after the rotation test under heating were also compared.

TABLE 1 Compara- Compara- Compara- Compara- Compara- Compara- Compara- tive tive tive tive tive tive tive Example Example Example Example Example Example Example 1 2 3 4 5 6 7 Base oil Ester oil TOTM + TOPM ◯ ◯ ◯ ◯ ◯ PAO 50 cSt ◯ ◯ Mineral oil — ◯ Thickener Diurea A Aromatic urea + ◯ ◯ ◯ ◯ ◯ aliphatic urea Diurea B Aliphatic urea + ◯ ◯ alicyclic urea Dispersing agent A Polyacrylic acid 1.0% sodium salt B Copolymer of 1.0% maleic acid sodium salt and isobutylene C sebacic acid 1.0% 0.5% 0.2% 1.0% 1.0% sodium salt Average particle diameter 10 7 6.0 6.0 6.0 6.0 6.0 of dispersing agent (μm) Initial Anderon value Measured value 1.8 1.7 1.5 1.3 1.1 1.5 1.5 of Anderon value in M band Anderon value after Measured value 2 1.7 1.8 — — 1.9 2 rotation test under heating of Anderon value in M band Viscosity after test of Pa · s 5710 5650 4320 6540 7630 4560 4750 heating and being left to stand Ex- Ex- Ex- Ex- Ex- Ex- Ex- ample 1 ample 2 ample 3 ample 4 ample 5 ample 6 ample 7 Base oil Ester oil TOTM + TOPM ◯ ◯ ◯ ◯ ◯ ◯ ◯ PAO 50 cSt Mineral oil — Thick-ener Diurea A Aromatic urea + ◯ ◯ ◯ ◯ ◯ ◯ ◯ aliphatic urea Diurea B Aliphatic urea + alicyclic urea Dispersing agent A Polyacrylic acid 1.0% sodium salt B Copolymer of 1.0% maleic acid sodium salt and isobutylene C sebacic acid 1.0% 1.0% 1.0% 1.0% 1.0% sodium salt Average particle diameter 5.0 3.0 3.0 2.4 1.5 1.4 0.9 of dispersing agent (μm) Initial Anderon value Measured value 0.6 0.6 0.6 0.4 0.20 0.17 0.16 of Anderon value in M band Anderon value after Measured value 0.95 0.85 0.75 — — — — rotation test under heating of Anderon value in M band Viscosity after test of Pa · s 3980 3950 3620 3320 3280 3140 3030 heating and being left to stand Ex- Ex- Ex- Ex- Ex- ample 8 ample 9 ample 10 ample 11 ample 12 Base oil Ester oil TOTM + TOPM ◯ ◯ ◯ PAO 50 cSt ◯ ◯ Mineral oil — ◯ Thick-ener Diurea A Aromatic urea + aliphatic urea ◯ ◯ ◯ Diurea B Aliphatic urea + alicyclic urea ◯ ◯ Dispersing agent A Polyacrylic acid sodium salt B Copolymer of maleic acid sodium salt and isobutylene C sebacic acid sodium salt 0.5% 0.2% 1.0% 1.0% 5.0% Average particle diameter 0.9 0.9 3.0 3.0 0.9 of dispersing agent (μm) Initial Anderon value Measured value of Anderon value 0.15 0.12 0.60 0.62 0.80 in M band Anderon value after Measured value of Anderon value — — 0.85 0.8 — rotation test under heating in M band Viscosity after test of Pa·s 3550 3880 3350 3380 3800 heating and being left to stand

The viscosity (unit: Pa·s) was measured with a rotation viscometer after carrying out the stationary heating test in which each of the grease compositions were left stationary under an environment of 160° C. for 500 hours. Specifically, the stationary heating test was carried out at a temperature condition of 160° C. for 500 hours to the prepared grease compositions and thereafter the viscosity was measured using a rheometer under conditions of a temperature of 25° C., a shear rate of 1/s, and a gap of 0.2 mm. This viscosity measurement means that as the viscosity becomes higher, the rotational torque of the rolling bearing filled with the grease composition becomes higher after the rolling bearing is used under high temperature. The results of the above experiments are listed in Table 1.

The initial Anderon value was measured after preparing rolling bearings filled with each of the grease compositions and rotating each of the rolling bearings at a rotation speed of 1800 rμm during 1 minute at room temperature. Specifically, each of the test grease was encapsulated in a steel shielded ball bearing (inner diameter 8 mm, outer diameter 22 mm and width 7 mm) in an amount between 25% and 35% of the bearing volume. The ball bearing was installed in a housing and a shaft was inserted into the bearing inner diameter, followed by coupling the shaft to the rotating shaft of a test motor and rotating the ball bearing at room temperature so that the ball bearing was rotated by the inner ring. After rotating each of the ball bearings for one minute, Anderon value of the ball bearing was measured using an Anderon Meter (manufactured by Sugawara Laboratories Inc.) in M band (300 Hz to 1800 Hz). This value was defined as the initial Anderon value. The frequency of the M band (300 Hz to 1800 Hz) is considered to be harsh sound for humans.

For the grease compositions of the Examples 1 to 7 and Comparative Examples 1 to 3, the viscosities were measured as described above in order to verify the effect of the average particle diameters of the dispersing agents to heat resistance. The experimental result is shown in FIG. 3. In FIG. 3, the horizontal axis represents the average particle diameter of the dispersing agent and the vertical axis represents the viscosity. According to the result illustrated in FIG. 3, after the stationary heating test under a high temperature environment of 160° C., the viscosities of the grease compositions having average particle diameters of the dispersing agents of 5 μm or less are significantly lower than the viscosities of the grease compositions having average particle diameters of the dispersing agents of 7 μm or more. In addition, the increasing rate of the viscosity in the range of the average particle diameter of the dispersing agents corresponding to 5 μm to 7 μm is larger than in the other ranges. Therefore, it was demonstrated that better heat resistance can be obtained by setting the average particle diameter of the dispersing agent to 5 μm or less.

Moreover, for the grease compositions of Examples 1 to 7 and Comparative Examples 1 to 3, the initial Anderon values were measured as described above. The experiment result is shown in FIG. 4. In FIG. 4, the horizontal axis represents the average particle diameter of the dispersing agent and the vertical axis represents the initial Anderon value. As illustrated in FIG. 4, the initial Anderon values of the grease compositions having average particle diameters of the dispersing agents of 5 or less is 0.6 or less, whereas the initial Anderon values of the grease compositions having average particle diameters of the dispersing agents of 6 μm or more is 1.5 or more. Therefore, it was confirmed that the use of the dispersing agent having an average particle diameter of 5 μm or less shows significantly low initial Anderon values and thus the acoustic property (quietness) is particularly excellent.

FIG. 5 is a graph in which the Anderon values (M band) of the ball bearings after the rotation test under heating are compared with the initial Anderon values. The rotation test under heating was performed with the ball bearings filled with each of the grease compositions of Examples 1 to 3, 10, and 11 and Comparative Examples 1 to 3, 6, and 7, where the ball bearing was rotated at a test temperature of 120° C. and a rotation speed of 3000 rpm for 500 hours.

As the ball bearing, a steel shielded ball bearing having the same dimension described above was used. Similarly, the test grease was also supplied in an amount between 25% and 35% of the bearing volume. In FIG. 5, the Anderon values after the rotation test under heating are higher than the initial Anderon values in any of Examples and Comparative Examples. The increased amounts, however, are not so large and thus it can be verified that the increase in the Anderon values is reduced by adding the dispersing agents. In Examples, both the initial Anderon values and the Anderon values after the rotation test under heating are less than 1.0. On the other hand, in Comparative Examples, the initial Anderon values and the Anderon values after the rotation test under heating are less than 2.0 but exceed 1.0. Consequently, it was confirmed that addition of the dispersing agent to the grease composition reduces deterioration of the Anderon value under high temperature and, among the grease compositions, the grease composition containing the dispersing agent having an average particle diameter of 5 μm or less can provide excellent acoustic property.

The embodiment of the present invention can provide a grease composition that causes less thermal deterioration when the grease composition is used under a high temperature environment, and a rolling bearing and a motor in which excellent acoustic property is maintained from the initial stage to the end of the service lifetime when the rolling bearing and the motor are used under the high temperature environment.

Although the invention has been described with respect to specific embodiments for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art that fairly fall within the basic teaching herein set forth. 

What is claimed is:
 1. A grease composition comprising: a base oil; a thickener; and 0.2 wt % or more of a dispersing agent, wherein the dispersing agent comprises at least one of a carboxylic acid metal salt, a metal salt of a polycarboxylic acid and a copolymer of a carboxylic acid metal salt and a hydrocarbon compound, and the dispersing agent has an average particle diameter of 0.5 μm or more and 5 μm or less.
 2. The grease composition according to claim 1, wherein at least one of the carboxylic acid metal salt and the metal salt of a polycarboxylic acid is an alkali metal salt including at least one of a sodium salt, a lithium salt and a potassium salt, or an alkali earth metal salt including at least one of a magnesium salt and a calcium salt.
 3. The grease composition according to claim 1, wherein the carboxylic acid metal salt forming the copolymer of the carboxylic acid metal salt and the hydrocarbon compound is an alkali metal salt including at least one of a sodium salt, a lithium salt and a potassium salt, or an alkali earth metal salt including at least one of a magnesium salt and a calcium salt.
 4. The grease composition according to claim 1, wherein the content of the dispersing agent is 5 wt % or less.
 5. The grease composition according to claim 1, wherein the thickener comprises a diurea compound represented by Formula (1): R₁—NHCONH—R₂—NHCONH—R₃   (1) where R₁, R₂, and R₃ are hydrocarbon groups selected from aliphatic hydrocarbon groups, alicyclic hydrocarbon groups and aromatic hydrocarbon groups.
 6. A grease composition comprising: a base oil; a thickener comprising a diurea compound represented by Formula (1): R₁—NHCONH—R₂—NHCONH—R₃   (1) where R₁, R₂, and R₃ are hydrocarbon groups selected from aliphatic hydrocarbon groups, alicyclic hydrocarbon groups and aromatic hydrocarbon groups; and 0.2 wt % or more and 5 wt % or less of a dispersing agent, wherein the dispersing agent comprises at least one of a sodium salt of sebacic acid, a sodium salt of polyacrylic acid, and a copolymer of a maleic acid sodium salt and isobutylene; and the dispersing agent has an average particle diameter of 0.5 μm or more and 5 μm or less.
 7. A rolling bearing comprising the grease composition according to claim
 1. 8. A rolling bearing comprising the grease composition according to claim
 6. 9. A motor comprising the rolling bearing according to claim
 7. 10. A motor comprising the rolling bearing according to claim
 8. 